Method and circuit arrangement for operating an electromagnetically actuated mechanical brake of an electric motor

ABSTRACT

Electric motors, in particular geared motors, are often equipped with electromagnetically actuated mechanical brakes. In order to ensure that an excitation coil ( 3 ) can be operated with alternating-current mains supplies of various voltages, it is proposed to generate a unipolar voltage that varies over time with a certain period by means of a voltage source ( 2 ), the output voltage of which is positive and different from zero in a first time segment of this voltage period and in this first time segment has a maximal value, and in a second segment of the voltage period is zero or approaches zero. To pass an excitation current from the voltage source ( 2 ) to the excitation coil ( 3 ), a current path ( 15 ) is provided that is opened substantially at the beginning of the first time segment. The current path ( 15 ) is blocked again at a certain point within the first time segment, this point being selected such that the excitation current does not exceed a predetermined maximal value. During a time span in which the excitation coil ( 3 ) is or should be permanently excited, the opening and blocking of the current path are repeated in each voltage period.

The invention relates to a method and a circuit arrangement for theoperation of an electromagnetically actuated mechanical brake of anelectric motor, in particular a geared motor.

BACKGROUND OF THE INVENTION

Electric machines, in particular electric motors, can be very wellcontrolled with respect to their rotational speed. One known means ofimproving this basic controllability is to provide mechanical brakes,which can be released or raised by means of an electromagneticapparatus. While the electric motor is in operation, the currentpowering the electric motor is also supplied to an excitation coil ofthe electromagnetically actuated mechanical brake. When no current isthus supplied, the electric motor is immobilized by application of aspring force to the brake.

Furthermore, a known means of powering electric motors is by a mainssupply of alternating or polyphasic current. In this case, if the motorincorporates an excitation coil operated by direct current, convertersor rectifiers are employed. Because the operating voltages of a.c. orpolyphasic mains vary in different parts of the world, this equipment ismanufactured and marketed in a number of variants, with excitation coilsfor different operating voltages. For the same reason, diverse rectifiercircuit arrangements are produced. In order to reduce costs byincreasing the production output, efforts are already being made tointroduce a rectifier circuit arrangement that can be operated inseveral voltage categories that are used globally. However, because theamplitude of the unipolar voltage generated by the circuit arrangementdepends on the a.c. voltage amplitude, the excitation coil of theelectromagnetically actuated mechanical brake must be selected to beentirely compatible with the mains voltage available at the site wherethe brake is to be used.

The object of the invention is to develop a method and a circuitarrangement for the operation of an electromagnetically actuatedmechanical brake of an electric motor, in particular a geared motor, insuch a way that the same type of excitation coil can be used with a.c.or three-phase mains supplies of different voltages.

SUMMARY OF THE INVENTION

The invention thus is concerned with a method for operating anelectromagnetically actuated mechanical brake for an electric motor, inparticular a geared motor, with an excitation coil to actuate the brake,in particular to raise it, and with an alternating-current mains supply,i.e. one that delivers a mains voltage. the amplitude of which variescyclically, wherein the excitation coil has a time constantcorresponding to a ratio of a coil inductivity and an ohmic coilresistance.

So that the brake can be operated with a broad range of mains voltages,a unipolar voltage that changes with a specified period is produced by avoltage source.

The unipolar voltage in a first time segment of its period is positiveand different from zero and in this first time segment has a maximalvalue, and in a second time segment of its period is zero or at leastapproaches zero.

A current path for an excitation current from the voltage source to theexcitation coil is opened substantially at the beginning of the firsttime segment.

The current path is subsequently blocked, at a particular moment withinthe first time segment which is specified to be such that the excitationcurrent does not exceed a predetermined maximal level.

During a time span in which the excitation coil is or should bepermanently excited, the current path is opened and blocked repeatedly,once in each voltage cycle.

Preferably the unipolar voltage is produced by a rectifier unit in thevoltage source that acts on a single-phase mains input from the a.c.mains supply, in such a way that the first time segment is equal toalmost the entire period of the a.c. cycle, so that the second segmentis extremely short, and the voltage period corresponds to half of thea.c. period.

Alternatively, the first and the second segment can each correspond tohalf of the voltage period, and half of the period of the a.c. cycle.

Preferably the current path is controlled by way of the main currentelectrodes of an electronic one-way valve that can be turned on and offby means of a controlled element, the current path being opened when theone-way valve is turned on and being blocked when the one-way valve isturned off. While the one-way valve is off, a discharge current from theexcitation coil passes through a free path parallel to the excitationcoil, with a decay time course corresponding substantially to the timeconstant of the excitation coil.

The current path is opened when the unipolar voltage, rising from zero,first reaches a positive threshold value which is considerably lowerthan the maximal value.

In a first embodiment of the invention the excitation current ismeasured and the current path is blocked when the excitation current, asit increases from lower values after the current path has been opened,first reaches a predetermined first threshold maximum.

In a second embodiment of the invention the unipolar voltage is measuredand an integral thereof is obtained as the unipolar voltage rises fromzero during the first time segment, after the current path has beenopened; the current path is blocked when this gradually increasingintegral value first exceeds a predetermined second threshold maximum,which is so specified that the excitation current does not exceed apredetermined maximal level.

In a third embodiment of the invention the unipolar voltage is measuredand, by way of a first-order delay element, a first weighted measure ofthe unipolar voltage is obtained; here the current path is blocked whenthis first weighted measure, as it increases from lower values after thecurrent path has been opened, for the first time exceeds a predeterminedthird threshold maximum. In this case a time constant of the delayelement is adjusted to substantially correspond to the duration of thefirst time segment and the third threshold maximum is set such that theexcitation current does not exceed a predetermined maximal value.

In a fourth embodiment of the invention a voltage that exists along theexcitation coil is measured and by way of a firstorder delay element aweighted measure of this voltage is obtained, the current path beingblocked when the second weighted measure, as it increases from lowervalues after the current path has been opened, for the first timeexceeds a predetermined fourth threshold maximum. Here a time constantof the first-order delay element is adjusted to substantially correspondto the time constant of the excitation coil, preferably being twice aslong as the period of the unipolar voltage, and the fourth thresholdmaximum is specified such that the excitation current does not exceed apredetermined maximal value. In this way an “image”, so to speak, of thecurrent flowing through the excitation coil is obtained, so that nodirect current measurement is needed. This makes the circuit lesselaborate.

In all of these embodiments the current flowing through the excitationcoil is adjusted to a value that corresponds to the requirements of thebrake.

Preferably in the first to fourth embodiments the preetermined thresholdmaximum is changed depending on the operating state of the brake, inparticular depending on whether the brake is being raised or is beingkept in the raised state, so that the operating state of the brake(being raised or held up) is taken into account. Preferably when thecurrent begins to flow for the first time in order to raise the brake,for a certain time span (the elevation interval) the predeteminedthreshold maximum is readjusted to a level higher that than for holdingthe brake up, so that the raising process is accelerated. This elevationinterval is preferably longer than or equal to the time constant of theexcitation coil or at least twice as long as the period of the unipolarvoltage.

When the one-way valve is used, to provide protection from transientovervoltages, regardless of other control signals the one-way valve isalways turned on whenever a voltage between its main current electrodes,as a result of the transient overvoltage, exceeds an overvoltage limitthat is higher than the maximal value of the unipolar voltage expectedin normal operation; as a result, the overvoltage is reduced by way ofthe excitation coil.

Furthermore, the one-way valve is turned on whenever a positive voltageis applied to its controlled element and is turned off whenever anegative or zero voltage is applied to the controlled element, thearrangement being such that to turn on the one-way valve the unipolarvoltage is sent to the controlled element by way of at least one ohmicresistance and the positive control voltage is preferably limited by abipolar transistor.

In both cases, to turn off the one-way valve the controlled element isshort-circuited by way of at least one ohmic resistance and anelectronic switch.

The elevation interval mentioned above, during which the current flow isincreased as the brake is being raised, is controlled by a timingelement in such a way that the elevation interval begins when, as aresult of the mains voltage from the a.c. mains being turned on to raisethe brake, the unipolar voltage for the first time reaches apredetermined threshold value that is considerably lower than themaximal value.

In the second embodiment of the invention the integral value during thesecond time segment is reset to zero.

In the fourth embodiment of the invention the first-order delay elementis connected to the voltage that exists along the excitation coil duringopening of the current path, and is short-circuited at its input side byway of an electronic switch during blockade of the current path.

With regard to the associated apparatus, the invention relates to acircuit arrangement for the operation of an electromechanically actuatedmechanical brake for an electric motor, in particular a geared motor, bymains supplies within a broad range of voltages, comprising

an excitation coil with terminal connectors, which has a time constantcorresponding to a ratio of a coil inductivity and an ohmic coilresistance, for the purpose of actuating and in particular raising thebrake;

an alternating-current mains supply, the voltage of which alternateswith an a.c. period;

a voltage source to produce a unipolar voltage that changes with acertain period, being positive and different from zero in a first timesegment of the voltage period, during which it has a maximal value, andbeing zero or at least approaching zero in a second time segment of theperiod;

a current path for an excitation current, which leads from the voltagesource to the excitation coil and back;

an electronic one-way valve with main current electrodes across whichthe current path leads so that the current path can be opened andblocked, and with a controlled element for turning the one-way valve onand off;

a control circuit connected to the controlled element and so constructedthat the one-way valve is turned on substantially at the beginning ofthe first time segment in order to open the current path and is turnedoff at a certain moment within the first time segment in order to closethe current path,

the control circuit further being so constructed that the excitationcurrent does not exceed a preset maximal value and during a time span inwhich the excitation coil is or should be permanently excited, theturning on and off of the one-way valve to open and block the currentpath is repeated in consecutive voltage cycles.

Preferably the voltage source is connected at its input side to asingle-phase voltage supply from the a.c. mains and comprises

a diode-bridge circuit, so that the first time segment comprisessubstantially the entire period of the a.c. cycle, so that the secondtime segment is very short and the voltage period correspondssubstantially to half the a.c. period.

Alternatively, the voltage source comprises a diode-midpoint circuit, sothat the first and the second time segment each correspond to half thevoltage period as well as half the a.c. period.

In addition, in parallel to the terminal connectors of the excitationcoil there is preferably connected a free-run path with a circuitarrangement to discharge the excitation coil after the current path hasbeen blocked, with a discharge time constant corresponding to the timeconstant of the excitation coil.

A voltage meter is preferably provided to monitor the unipolar voltage,as well as a threshold element to preset a positive threshold value,such that the controlling device turns on the one-way valve to open thecurrent path whenever the unipolar voltage, rising from zero, firstreaches the positive threshold value, which is considerably lower thanthe maximal value.

A first embodiment of the invention comprises a current meter to producea current signal that corresponds to the excitation current, and athreshold element to preset a first threshold maximum, such that thecontrol circuit turns off the one-way valve to block the current pathwhen the current signal, as it rises from lower values after the currentpath has been opened, first reaches the first threshold maximum.

A second embodiment of the invention comprises an integrator element toform an integral of the unipolar voltage as it rises from zero duringthe first time segment and a threshold element to preset a secondthreshold maximum, which is adjusted so that the excitation current doesnot exceed a preset maximum value; in this embodiment the integral valueand the second threshold maximum are sent to the control circuit, whichturns off the one-way valve to block the current path when the integralvalue, as it rises from lower integral values after the current path hasbeen opened, first reaches the second threshold maximum.

A third embodiment of the invention comprises a voltage meter to monitorthe unipolar voltage and to generate a voltage measurement correspondingto the unipolar voltage, a first-order delay element with a timeconstant that corresponds substantially to the duration of the firsttime segment, to produce a weighted measure from the voltagemeasurement, and a threshold element to preset a third threshold maximumso adjusted that the excitation current does not exceed a predeterminedmaximum, in which the control circuit turns off the one-way valve toblock the current path when the weighted measure, as it rises from alower weighted measure after the current path has been opened, firstreaches the third threshold maximum.

A fourth embodiment of the invention comprises a voltage meter tomonitor a voltage at the terminal connectors of the excitation coil andto produce a corresponding terminal-voltage measurement, a first-orderdelay element with a time constant the corresponds substantially to thetime constant of the excitation coil and preferably is at least twice aslong as the period of the unipolar voltage, to produce a weighted secondmeasure from the terminal-voltage measurement, and a threshold elementto preset a fourth threshold maximum so adjusted that the excitationcurrent does not exceed a predetermined maximum, wherein the controlcircuit turns off the one-way valve to block the current path when theweighted second measure, as it rises from a lower weighted secondmeasure after the current path has been opened, first reaches the fourththreshold maximum. In this embodiment a “model” of the excitationcurrent is provided by a voltage measurement.

Preferably in these four embodiments the control circuit includesdevices to alter the threshold maximum in dependence on an operatingstate of the brake, namely raising the brake or keeping it raised, so asto take the operating state of the brake into account. For this purposethe control circuit comprises a timing element by means of which, whencurrent is beginning to flow for the first time in order to raise thebrake, the threshold maximum is set at a higher level than when thebrake is being held in a raised condition. Thus the dynamic behaviour ofthe brake is improved and the amount of heating is reduced.

An overvoltage protection device is provided, which monitors a voltagebetween the main current electrodes and, in order to protect the one-wayvalve from transient voltages, always turns the one-way valve on whenthe voltage transiently exceeds an overvoltage limit, which is greaterthan the maximal value of the unipolar voltage expected during normaloperation.

The circuit arrangement further comprises a protective device that turnsthe one-way valve on whenever a positive voltage exists at thecontrolled element, and turns it off whenever a negative or zero voltageexists at the controlled element. This circuit arrangement comprises atleast one ohmic resistance to connect the controlled element with theunipolar voltage to turn on the one-way valve and a limiting bipolartransistor in parallel to the controlled element, to limit a positivecontrol voltage.

Preferably at least one ohmic resistance and one electronic switch areprovided, to short-circuit the controlled element when the one-way valveis turned off.

The timing element comprises at least one ohmic resistance through whicha voltage supply is obtained from the unipolar voltage.

The second embodiment comprises at least one ohmic resistance and onecapacitor to form the integrator device, at least one ohmic resistanceto connect the integrator device with the unipolar voltage during thefirst time segment, and an electronic switch plus an ohmic resistance toreset the integrator device to zero in the second time segment.

In the third embodiment the first-order delay element comprises at leastone ohmic resistance and a capacitor and is connected to the unipolarvoltage by at least one ohmic resistance.

The fourth embodiment comprises at least one ohmic resistance and acapacitor to form the first-order delay element, at least one ohmicresistance to connect the first-order delay element to the unipolarvoltage when the current path is open, and an electronic switch toshort-circuit the first-order delay element at its input side when thecurrent path is blocked.

The preferred implementation of the valve is a switching transistor,preferably an IGBT (Insulated Gate Bipolar Transistor). An IGBT iscapable of switching large currents when the operating voltages are highand can be triggered by simple means.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is explained by description of exemplaryembodiments. However, the invention is not limited to these exemplaryembodiments. Their description is assisted by the attached drawings,wherein

FIG. 1 is the basic circuit diagram of a first exemplary embodiment ofthe circuit arrangement in accordance with the invention,

FIG. 2 is the basic circuit diagram of a second exemplary embodiment,

FIG. 3 is the circuit diagram of an exemplary embodiment with acomparator circuit,

FIG. 4 is the circuit diagram of an exemplary embodiment with athyristor circuit,

FIG. 5 is a diagram representing the time course of a unipolar voltageand of the excitation current,

FIG. 6 is the basic circuit diagram of another exemplary embodiment ofthe circuit arrangement in accordance with the invention,

FIG. 7 is the circuit diagram of an arrangement to make available aunipolar voltage,

FIG. 8 is a diagram showing the time course of a unipolar voltage thatcomes from the circuit according to FIG. 7,

FIG. 9 is the circuit diagram of another exemplary embodiment to producea unipolar voltage,

FIG. 10 is a diagram showing the time course of the unipolar voltagefrom the circuit according to FIG. 9,

FIG. 11 is the basic circuit diagram of another exemplary embodiment ofthe circuit arrangement in accordance with the invention,

FIG. 12 is the basic circuit diagram of yet another exemplary embodimentof the circuit arrangement in accordance with the invention,

FIG. 13 is the basic circuit diagram of a final exemplary embodiment ofthe circuit arrangement in accordance with the invention,

FIG. 14 is the circuit diagram of part of an exemplary embodiment withovervoltage protection for the one-way valve,

FIG. 15 is the circuit diagram of another embodiment of the arrangementaccording to FIG. 14,

FIG. 16 is a partial circuit diagram to illustrate a first-order delayelement,

FIG. 17 is a circuit diagram of an exemplary embodiment in which thecurrent path is blocked in dependence on the voltage at the excitationcoil,

FIG. 18 is the circuit diagram of an arrangement to produce an elevatedvoltage for the turning-on process, and

FIG. 19 is the circuit diagram of another arrangement to implement anelevated turn-on voltage.

DETAILED DESCRIPTION

In the following description, the same reference numerals are used foridentical parts or parts with identical actions.

In the basic circuit diagram of FIG. 1 the reference numeral 1 indicatesan alternating-current or three-phase mains supply, which is applied toa voltage source 2, the rectified unipolar output voltage U_(G) of whichis used to operate the excitation coil 3. A current path 15 for theexcitation current connects the output terminals of the voltage source 2to one another. In the current path 15 there are connected in series theexcitation coil 3, a valve 4 which is constructed as a one-way valve,and a current sensor 5. Also coupled to the output terminals of thevoltage source 2 is the control circuit 6, by means of which the valve 4can be made non-conducting in dependence on a measurement signalproduced by the current sensor 5. The excitation coil 3 is disposed inparallel to a free-run diode 7, through which the excitation current canflow when the valve 4 is shut off because of the inductivity of theexcitation coil 3. By this means extreme switching peaks can be avoided.The circuit arrangement thus formed is manufactured as a structural uniteither with or without the excitation coil 3.

The circuit arrangement shown in FIG. 2 differs from that according toFIG. 1 inasmuch as the former circuit arrangement includes twoadditional components. In this circuit the excitation coil 3 isconnected in parallel with an arrangement in which the free-run diode 7is connected in series with a varistor 8, the varistor 8 in turn beingconnected in parallel with a switch contact 9. To shorten the brakeapplication time, i.e. for the rapid operation of an electromagneticallyactuated mechanical brake, not only is the a.c. or three-phase mainssupply disconnected from the voltage source 2 but, in addition, theexcitation coil 3 is turned off directly by the switch contact 9. Forits own protection the switch contact 9 is connected parallel to thevaristor 8. The turn-on voltage of the varistor 8 is preferably adjustedto be somewhat greater than the peak value of the unipolar voltageU_(G).

With reference to FIG. 5 an exemplary embodiment of the method ofadjusting the excitation current of an excitation coil in accordancewith the invention will now be described. Reference will also be made tothe basic circuit diagram shown as an example in FIG. 1. At an earliertime, not shown in FIG. 5, the (a.c.) supply voltage is turned on, sothat from this moment on there appears at the output terminals of thevoltage source 2 a secondary, unipolar voltage UG that changesperiodically in time.

At the beginning of the first cycle of the unipolar voltage U_(G) thecontrol circuit 6 puts the valve 4 into an electrically conductingstate, so that a certain portion of the unipolar voltage U_(G),determined by the internal resistance of the excitation coil 3 and theadditional resistances along the current path 15, is applied to theexcitation coil. Attenuated by the inductivity of the excitation coil 3,the excitation current I_(E) initially rises with the unipolar voltageU_(G). Depending on the dimensioning of the components of the circuitarrangement 10, the excitation current I_(E) can also continue to riseeven after the time at which the peak of the unipolar voltage U_(G) isreached. After a few cycles, at the latest, the excitation current I_(E)has risen so far that its value averaged over one cycle remainsapproximately constant.

In particular, given that the time constant τ_(B) is formed by the ratioof the inductivity and the ohmic resistance R_(B) of the excitation coil3, and is preferably larger than the unipolar voltage period, theexcitation current I_(E) reaches the maximal level I_(max) when theaverage over time of the voltage at the excitation coil 3 is larger thanthe product of the ohmic resistance R_(B) of the excitation coil 3 and apredetermined maximal value I_(max) of the excitation current I_(E). Inthe case shown in FIG. 5 this occurs at the moment t1. By way of themeasurement signal transmitted to it by the current sensor 5, thecontrol circuit 6 detects this event. The control circuit 6 thereuponturns off the valve 4, as a result of which the current path 15 isblocked. The excitation current I_(E) is then commutated into thefree-run diode 7 and as a result declines. This process continues untilthe beginning of the following cycle of the unipolar voltage U_(G).Thereafter, at the moment t2, the control circuit 6 turns the valve 4 onagain, so that the current path 15 is opened and the excitation currentI_(E) can rise again. The average over time of the excitation currentI_(E) is therefore at least slightly lower than the maximal currentI_(max), The moment t2 preferably occurs as soon as possible after thetime at which the unipolar voltage U_(G) reaches its minimal value(U_(min)).

If the peak value of the unipolar voltage U_(G) is smaller than theproduct of the ohmic resistance R_(B) and the maximal current I_(max),the excitation current I_(E) does not reach the level I_(max).Nevertheless, certain designs of the circuit in accordance with theinvention make it possible for the valve 4 to be closed temporarily evenin this case, namely within a brief interval during which the unipolarvoltage U_(G) nearly disappears. This time interval, however, as a ruleis considerably shorter than the period of the unipolar voltage U_(G).

The exemplary embodiment of a circuit arrangement in accordance with theinvention shown in FIG. 3 comprises a control circuit 6 with acomparator circuit 53. The control circuit 6 further comprises a controlresistor 162 and a Zener diode 163. The Zener diode 163 is connectedparallel to the controlled element of an IGBT (Insulated Gate BipolarTransistor) 41 and hence prevents the application of unacceptably highvoltages to this controlled element. On the other hand, the Zener diode163 is chosen to be such that current can be reliably passed throughthis controlled element. As switch element the comparator circuit 53includes an operational amplifier 642, which triggers a bipolartransistor 643, the collector of which is connected to one terminal ofthe voltage source 2 by way of the control resistor 162, the emittorbeing connected to the other terminal of the voltage source 2. Thecontrol resistance 162 is made small enough that the controlled elementof the IGBT 41 conducts reliably when the voltage U_(G) has even a smallvalue, preferably at least an order of magnitude smaller (factor of 0.1)than the peak value of the unipolar voltage U_(G). In the case of aunipolar voltage U_(G) that changes periodically in time, therefore, theIGBT 41 is put into the electrically conducting state at the verybeginning of a cycle.

To serve as the actuating variable, the control voltage U_(T) across thesensor resistance 5 is made available to the control circuit 6. Aportion of the control voltage U_(T) is compared, by the comparatorcircuit 53, with a reference voltage generated by a reference-voltagesource 12. The reference voltage is a measure of the maximal valueI_(max) of the excitation current. When the excitation current I_(E)reaches the maximal value I_(max), the control voltage U_(T) equals thereference voltage. Then the comparator circuit 53 flips, so that thebipolar transistor 643 becomes conducting and the IGBT 41 is put intothe electrically non-conducting state. The excitation current I_(E) isthen commutated into the free-run diode 7. Preferably the referencevoltage produced by the reference-voltage source 12 can be adjustedexternally, for example by way of an adjustment device to control thebraking behaviour that is not shown here.

A hysteresis resistor 641 between the non-inverting input of theoperational amplifier 642 and the point at which the excitation coil 3is connected to the IGBT 41 prevents the comparator circuit 53 fromflipping back as long as the unipolar voltage U_(G) has not yet returnedto a very small value, which preferably is at least an order ofmagnitude smaller than the peak value of the unipolar voltage U_(G).

In the circuit arrangement shown in FIG. 4 the control circuit 6 takesover the triggering of the IGBT 41. Here the maximal current I_(max) isderived from the dimensioning of the components of the control circuit6, of the IGBT 41 and of the sensor resistor 5. The control resistor162, connected in series with a thyristor 161 between the terminals ofthe voltage source 2, is chosen such that the controlled element of theIGBT 41 is made reliably conducting even when the unipolar voltage U_(G)has reached only a very small value, at least an order of magnitudesmaller than the peak value of the unipolar voltage U_(G). The Zenerdiode 163 connected in parallel with the controlled element of the IGBT41 is chosen such that on one hand the controlled element of the IGBT 41is made reliably conducting, whereas on the other hand an unacceptablyhigh voltage load on this controlled element is avoided. The value ofthe sensor resistor 5 is chosen such that the voltage U_(T) across itcorresponds to the gate voltage of the thyristor 161, when a currentwith the value of the maximal current I_(max) is flowing through thesensor resistor 5. In this case the thyristor 161 is triggered, i.e. itis put into an electrically conducting state, with the consequence thatthe connections to the controlled element of the IGBT 41 areshort-circuited. As a result, the IGBT 41 blocks the current path 15.The thyristor 161 does not become non-conducting until the unipolarvoltage U_(G) has fallen to 0. The IGBT 41 is therefore not turned onagain, opening the current path 15, until approximately the time whenthe unipolar voltage U_(G) begins to increase again.

In the basic circuit diagram shown in FIG. 6, in contrast to thoseaccording to FIGS. 1 and 2, no current sensor is provided. Nevertheless,the object stated above is also achieved by this circuit, namely byadjusting the duration of the time segment during which current issupplied to the excitation coil 3. This is explained in detail in thefollowing examples.

First, reference is made to FIGS. 7 and 8, which show an embodiment ofthe invention in which the voltage source 2 comprises a diode 51 in thecurrent path 15, so that a half-wave rectification of the alternatingcurrent coming from the a.c. mains supply 1 is accomplished. Between theterminals of the a.c. mains 1 a varistor 50 is disposed for protectionagainst overvoltages.

The voltage and current time courses are shown in FIG. 8. The diode 51allows only the positive half-cycle of the alternating current to beconducted, so that the unipolar voltage U_(G) is present only betweentime 0 and time t_(A). During the time t_(A) to T, i.e. during thesecond part of the a.c. period 0-T, no voltage is present. In thediagram of FIG. 8 a lower threshold U_(G0) and the maximal valueU_(Gmax) of the unipolar voltage U_(G) are also shown.

The excitation current I_(E), rises from the time at which the unipolarvoltage U_(G) exceeds the lower threshold U_(G0) until a time t1 atwhich the excitation current I_(E) reaches a maximal value I_(max); thenit decays until the unipolar voltage U_(G) again reaches the lowerthreshold U_(G0) during the next cycle T-2T. The cycle ofexcitation-current flow thus corresponds in duration to a full cycle ofthe unipolar voltage U_(G) but is somewhat shifted in time.

In the embodiment of the voltage source 2 shown in FIG. 9 four diodes51, 51′, 51″ and 51′″ are provided in a bridge arrangement, so that theunipolar voltage U_(G) has the time course shown in FIG. 10, as is knownper se. Here, too, the excitation current I_(E) rises at the time whenthe unipolar voltage U_(G) crosses the lower threshold U_(G0), and itlikewise continues to rise until the time t1 at which the excitationcurrent I_(E) reaches the maximal value I_(max), thereafter declininguntil the time at which the second half-wave of the unipolar voltageU_(G) crosses the lower threshold U_(G0). The period in this case ishalf as long as that in the exemplary embodiment presented above.

In the following, various embodiments of the control circuit areexplained with reference to block diagrams.

The embodiment shown in FIG. 11 comprises a voltage sensor 54, whichmonitors the unipolar voltage U_(G). The value thus measured is sent tothe input of an integrator device 52 and also to the input of a resetswitch 55, the output of which is connected to a reset input of theintegrator device 52. When the unipolar voltage U_(G) sensed by thevoltage sensor 54 falls below a predetermined level, the reset switch 55returns the value stored in the integrator device 52 to zero.

The output of the integrator device 52 is sent to a comparator 53, whichcompares the integral over time of the unipolar voltage U_(G), as it isformed in the integrator device 52, with a predetermined threshold valueF_(max). The result of this comparison is sent to a trigger circuit 56(corresponding to a Schmitt trigger), the output of which is applied tothe controlled element of the electronic one-way valve 4. If theintegral value exceeds the threshold maximum F_(max), the one-way valve4 is turned off.

In the following the function of this arrangement is described, withreference to the time course of the unipolar voltage U_(G) as shown inFIG. 8 or FIG. 10.

When the voltage U_(G) is zero (time 0), the reset switch 55 keeps thevalue of the integral in the integrator device 52 at zero. Now, when thevoltage rises, the reset switch 55 releases the reset input of theintegrator device 52, so that the device integrates the voltage U_(G)over time. The integral value is compared with the threshold maximumF_(max). When the integral value reaches the threshold maximum, as isthe case at the time designated t1 in FIG. 8 and FIG. 10, the valve 4,which was previously in the on state, is turned off, so that the currentpath 15 is blocked. At the time t_(A), at which the unipolar voltageU_(G) reaches the value zero, the integral value in the integratordevice 52 is returned to zero, and it is kept there until the voltageU_(G) rises again, so that this process begins anew.

The embodiment of the invention shown in FIG. 12 differs from thataccording to FIG. 11 in that instead of the integrator device 52 withthe reset switch 55, a first-order delay element 57 (low-pass) isprovided by which the unipolar voltage U_(G) is weighted. The currentpath 15 is closed, by turning off the valve 4, when the weighted valueof the unipolar voltage U_(G) exceeds a threshold maximum U_(max). Thisthreshold maximum U_(max), like the threshold maximum F_(max) in theprevious embodiment, is set such that the time t1 at which the valve isturned off coincides with the time at which the maximum of theexcitation current I_(max) is reached.

The arrangement shown in FIG. 13 differs from that according to FIG. 12in that not the unipolar voltage U_(G), but rather the voltage U_(E)across the terminals of the excitation coil 3 is measured and sent tothe delay element 57. In other respects the function is the same. If thetime constant of the first-order delay element 57 is chosen to be thesame as the time constant τ_(E) of the excitation coil 3, the timecourse of the output voltage of the first-order delay element 57 atpoint S corresponds exactly to that of the excitation current I_(E)—thatis, the voltage is an accurate representation of the current.

In the circuit of FIG. 14 details of the comparator 53, the triggercircuit 56 and the one-way valve 4 are shown. The point S in FIG. 14 canthus be joined to the corresponding point S in the arrangement accordingto one of the FIGS. 11 to 13, so that the symbolically indicated circuitcomponents 53 and 56 are realized by the circuit according to FIG. 14.

The point S is connected to a Zener diode 65 and a resistor 66 arrangedin series. The connection point between the Zener diode 65 and theresistor 66 is connected to the gate of a thyristor 61, which isconnected in series with a resistor 62 across the terminals of thevoltage source 2. The connection point between the thyristor 61 and theresistor 62 is connected, by way of a resistor 64, to the gate of avalve 41, which in turn is connected to the emitter of the valve by wayof a Zener diode 63 and to its collector by way of a Zener diode 67 inseries with a diode 68. The valve 41 corresponds to the valve 4 in FIGS.11 to 13.

The valve 41 is turned on when the unipolar voltage U_(G) first reachesthe value U_(G0). The valve 41 turns off when the signal at the input S(coming from the integrator device 52 or the first-order delay element57) reaches the threshold value (U_(max) or F_(max), respectively)determined by the sum of the trigger voltage of the thyristor 61 and theZener voltage of the Zener diode 65. Having been triggered, thethyristor 61 remains conducting regardless of the signal at S, until theunipolar voltage U_(G) has returned to zero. The diode 68 and the Zenerdiode 67 provide overvoltage protection for the valve 41, as follows. Atvoltages above a certain level the Zener diode 67 becomes conducting,thus allowing the current to bypass the valve 41.

The Zener diode 63 in turn protects the controlled element of the valve41.

The circuit shown in FIG. 15 serves substantially the same function asthe circuit of FIG. 14. The latching mechanism, corresponding to theproperty of the thyristor 61 that it remains conducting after havingbeen triggered, in the circuit of FIG. 15 consists in theself-maintaining mechanism of a transistor 611 provided in place of thethyristor 61 shown in FIG. 14. This behaviour is achieved by includingin the circuit according to FIG. 15 the resistor 69, which connects thepoint S to the collector of the valve 41. When the voltage at point Sreaches the threshold value (U_(max), F_(max)) defined by the Zenerdiode 65 together with the base voltage of the transistor 611, thetransistor 611 turns on. As a result, the base voltage of the transistor41 is drawn down, so that the transistor 41 becomes non-conducting. Thenits collector is substantially at the unipolar voltage U_(G) (ignoring,for the moment, the voltage drop across the excitation coil 3).Therefore an additional control current for the transistor 611 isobtained from the collector of the transistor 41, by way of the resistor69 and the Zener diode 65.

The circuit according tb FIG. 16 comprises a series arrangement of aresistor 71 and a capacitor 73, the latter being connected parallel to aresistor 72. The connection point between the resistor 71 and thecapacitor 73 is identified in FIG. 16 as point S. This circuitconstitutes a simple form of an integrator device 52, so that the pointS can be joined to the similarly designated point S in one of thecircuits according to FIG. 14 or FIG. 15, to form a complete controlcircuit 6. The capacitance of the capacitor 73 and the resistance of theresistors 71 and 72 are chosen to be such that the delay element soformed has a time constant of ca. 10 ms; as the voltage begins to risein each a.c. cycle, this element behaves approximately like anintegrator.

The variant of the invention shown in FIG. 17 involves a circuit inwhich the current flowing through the excitation coil 3 is accuratelyrepresented by a voltage obtained by suitable dimensioning of thefirst-order delay element formed by the resistor 71 and the capacitor 73(which is disposed in parallel with the resistor 72); this voltageU_(EV) appears at the point S. The valve 41 turns on (opening thecurrent path 15) when the voltage U_(G) is applied to the excitationcoil 3. The voltage U_(EV) (at point S) follows with a delay. When thevalve 41 turns off, the voltage at the excitation coil 3 is zero, andagain the voltage U_(EV) follows with a delay. For this to occur, theresistance 71 in the first-order delay element of FIG. 17 must be verymuch larger than the resistance 72, which in turn must be larger thanthe resistance 74. The remaining elements of the circuit can be seendirectly in FIG. 17.

In the circuit shown in FIG. 18 a turn-on elevation is implemented,namely an increase in the excitation current I_(E) that flows throughthe excitation coil 3 while the brake is being lifted, in comparisonwith that flowing while the brake is kept in the raised state. Toproduce this transient increase, the action of the first-order delayelement 57 that weights the measured value of the voltage U_(G) isreduced during a time interval T_(E) (which coincides with the liftingprocess), by turning on an auxiliary transistor 77 constructed as aself-conducting transistor. This transistor 77 is disposed in serieswith a resistor 76 to form a branch connected parallel to the capacitor73 and the resistor 72.

FIG. 19 shows a circuit to produce the signal that controls thetransistor 77 according to FIG. 18. With the Zener diode 93 and thecapacitor 92 shown in FIG. 19, a stabilized d.c. voltage U_(H) isproduced. The time interval T_(E) mentioned above is determinedsubstantially by the capacitance of the capacitor 81 and the resistanceof the resistor 82. Initially the connection point between the capacitor81 and the resistor 82 is nearly at the level of U_(H). From thecollector of the transistor 84 current flows to the base of thetransistor 85 and hence to its collector. As a result, the transistors84 and 85 become conducting. The voltage U_(TE) at point E thus becomesapproximately zero, so that the transistor 77 (see FIG. 18) becomesconducting. Now when the capacitor 81 is charged to substantially thevoltage U_(H), by way of the resistor 82, the connection point betweenthe capacitor 81 and the resistor 82 is nearly at zero voltage.Therefore no more current flows through the resistor 82 to the collector84, and hence the transistors 84 and 85 become non-conducting, so thatthe transistor 77 is also non-conducting. The voltage U_(TE) risestowards the level U_(H) by way of the voltage divider formed by theresistors 86 and 87. The diode 83 discharges the capacitor 81 when theunipolar voltage U_(G) has been zero for a long time, i.e. for many a.c.cycles. Hence the circuit according to FIG. 19 exhibits bistablebehaviour.

The above descriptions make it evident that someone skilled in the artcan implement the method described at the outset with a variety ofcircuit designs. For industrial production, however, it is desirable toemploy the simplest and most economical variants with which therelatively high manufacturing accuracy of capacitors, resistors andZener diodes can be relied upon to achieve precise characteristic timeconstants, and in which the circuitry is not too laborious or costly toconstruct.

On the basis of the circuit arrangement in accordance with the inventionand/or the method in accordance with the invention, it is now possibleto operate the same type of excitation coil with a wide range of supplyvoltages. As a result, a given type of excitation coil can bemanufactured in greater numbers. This large-scale manufacture isadvantageous with respect to the costs of development, raw materials andtools, processing and testing equipment, and the logistics of storageand marketing.

What is claimed is:
 1. Method for the operation of anelectromagnetically actuated mechanical brake of an electric motor, inparticular a geared motor, with an excitation coil for the actuation, inparticular lifting of the brake, with an a.c. mains supply, the voltageof which alternates cyclically with an a.c. period (T_(W)), such thatthe excitation coil has a time constant (□B) corresponding to a ratio ofa coil inductivity and an ohmic coil resistance, wherein to operate thebrake with mains voltages within a wide voltage range a unipolar voltage(U_(G)) that changes with a period (T) is generated by a voltage source,the unipolar voltage (U_(G)) in a first time segment (0 to t_(A)) of thevoltage period (T) is positive and different from zero and in this firstsegment (0 to t_(A)) has a maximal value (U_(Gmax)), and in a secondtime segment (t_(A) to T) of the voltage period (T) is zero or at leastapproaches zero, a current path is opened for an excitation current toflow from the voltage source to the excitation coil substantially at thebeginning of the first time segment (0 to t_(A)), the current path isblocked again at a certain point in time (t₁≦t_(A)) within the firsttime segment (0 to t_(A)) the certain point in time (t₁≦t_(A)) beingchosen such that the excitation current does not exceed a predeterminedmaximal value (I_(max)), and wherein during a time interval in which theexcitation coil is or should be permanently excited, opening andblocking of the current path is repeated in each voltage cycle (T). 2.Method according to claim 1, wherein the unipolar voltage U_(G) isgenerated by a rectifier unit in the voltage source from a single-phasemains voltage of the a.c. mains, in such a way that the first timesegment (0 to t_(A)) coincides with substantially all of the a.c. period(T_(W)), so that the second segment (t_(A) to T) is very short, andwherein the voltage period (T) corresponds to half the a.c. period(T_(W)).
 3. Method according to claim 1, wherein the first and thesecond segments (0 to t_(A), t_(A) to T) each correspond to half thevoltage period (T) and half the a.c. period (T_(W)).
 4. Method accordingto claim 1, wherein the current path is opened or blocked by way of maincurrent electrodes of an electronic one-way valve that can be turned onand off by means of a controlled element, the current path being openedwhen the one-way valve is turned on and blocked when it is turned off,and wherein when the one-way valve is turned off, a discharge currentfrom the excitation coil is conducted through a free path parallel tothe excitation coil with a decay time course corresponding substantiallyto the time constant (τ_(B)) of the excitation coil.
 5. Method accordingto claim 4, wherein to protect against transient overvoltages theone-way valve is always turned on, regardless of other control signals,when a voltage between its main current electrodes, during the transientovervoltage, exceeds an overvoltage limit (U_(Grenz)) that is greaterthan the maximal value (U_(Gmax)) of the unipolar voltage (U_(G))expected during normal operation.
 6. Method according to claim 4,wherein the one-way valve is turned on when a positive voltage isapplied to its controlled element and is turned off when the voltage atthe controlled element is negative or zero.
 7. Method according to claim6, wherein in order to turn on the one-way valve the unipolar voltage(U_(G)) is applied to its controlled element by way of at least oneohmic resistance and the positive control voltage is preferably limitedby a bipolar transistor.
 8. Method according to claim 6, wherein inorder to turn off the one-way valve the controlled element isshort-circuited by way of at least one ohmic resistance and anelectronic switch.
 9. Method according to claim 1, wherein the currentpath is opened when the unipolar voltage (U_(G)), as it rises from zero,first reaches a positive threshold value (U_(G0)), which is considerablylower than the maximal value (U_(Gmax)).
 10. Method according to claim1, wherein the excitation current is measured and the current path isblocked when the excitation current, as it rises from lower currentlevels after the current path has been opened, for the first timereaches a first threshold maximum (I_(max)).
 11. Method according toclaim 10, wherein the predetermined threshold maximum (I_(max)) ischanged in dependence on an operating state of the brake, in particularin dependence on a raising of the brake and on its being kept in theraised state.
 12. Method according to claim 11, wherein at the beginningof an initial flow of current provided to raise the brake, thepredetermined threshold maximum (I_(max), F_(max), U_(max)) is set at alevel higher than that for keeping the brake raised, after which itremains at the higher level for an elevation interval (T_(E)). 13.Method according to claim 12, wherein the elevation interval (T_(E)) isgreater than or equal to the time constant (τ_(B)) of the excitationcoil.
 14. Method according to claim 12, wherein the elevation interval(T_(E)) is at least twice as long as the period (T) of the unipolarvoltage (U_(G)).
 15. Method according to claim 12, wherein the elevationinterval (T_(E)) is controlled by a timing element in such a way thatthe elevation interval (T_(E)) begins when, as a result of switching onthe mains voltage from the a.c. mains supply in order to raise thebrake, the unipolar voltage (U_(G)) first reaches a predeterminedthreshold value (U_(GI)) that is considerably lower than the maximalvalue (U_(Gmax)).
 16. Method according to claim 1, wherein the unipolarvoltage (U_(G)) is measured and an integral value of the unipolarvoltage (U_(G)) is obtained as it rises from zero during the first timesegment (0 to t_(A)), and wherein the current path is blocked when theintegral value, as it rises from lower integral values after the currentpath has been opened, for the first time reaches a predetermined secondthreshold maximum (F_(max)), the second threshold maximum (F_(max))being chosen such that the excitation current does not exceed apredetermined maximum (I_(max)).
 17. Method according to claim 16,wherein the integral value is reset to zero during the second timesegment (t_(A) to T).
 18. Method according to claim 16, wherein thepredetermined threshold maximum (F_(max)) is changed in dependence on anoperating state of the brake, in particular in dependence on a raisingof the brake and on its being kept in the raised state.
 19. Methodaccording to claim 1, wherein the unipolar voltage (U_(G)) is measuredand by means of a first-order delay element a first weighted measure ofthe unipolar voltage (U_(G)) is obtained, and wherein the current pathis blocked when the first weighted measure, as it rises from lowervalues of this weighted measure after the current path has been opened,for the first time reaches a predetermined third threshold maximum(U_(max)), a time constant of the delay element being set to correspondsubstantially to the duration of the first time segment (0 to t_(A)) andthe third threshold maximum (U_(max)) being chosen such that theexcitation current does not exceed a predetermined maximum (I_(max)).20. Method according to claim 19, wherein the predetermined thresholdmaximum (U_(max)) is changed in dependence on an operating state of thebrake, in particular in dependence on a raising of the brake and on itsbeing kept in the raised state.
 21. Method according to claim 1, whereina voltage (U_(E)) along the excitation coil is measured and by means ofa first-order delay element a weighted second measure of this voltage isobtained, and wherein the current path is blocked when the secondmeasure, as it rises from lower values after the current path has beenopened, for the first time reaches a predetermined fourth thresholdmaximum (U_(max)), a time constant of the first-order delay elementbeing set to correspond substantially to the time constant (□_(B)) ofthe excitation coil and being preferably at least twice as long as theperiod (T) of the unipolar voltage (U_(G)), and the fourth thresholdmaximum (U_(max)) being chosen such that the excitation current does notexceed a predetermined maximum (I_(max)).
 22. Method according to claim21, wherein the first-order delay element is connected to the voltage(U_(G)) along the excitation coil while the current path is opened, andis short-circuited at its input side by way of an electronic switchwhile the current path is blocked.
 23. Method according to claim 21,wherein the predetermined threshold maximum (U_(max)) is changed independence on an operating state of the brake, in particular independence on a raising of the brake and on its being kept in the raisedstate.
 24. Circuit arrangement for the operation of anelectromechanically actuated mechanical brake of an electric motor, inparticular a geared motor, with mains voltages over a wide voltagerange, comprising an excitation coil with terminal connectors, which hasa time constant (τ_(B)) corresponding to a ratio between a coilinductivity and an ohmic coil resistance, to actuate, in particular toraise, the brake; an alternating-current mains supply that delivers amains voltage with an a.c. period (T_(W)); a voltage source to generatea unipolar voltage (U_(G)) that changes with a period (T), which in afirst time segment (0 to t_(A)) of the voltage period (T) is positiveand different from zero and in this first time segment (0 to t_(A)) hasa maximal value (U_(Gmax)) and which in a second time segment (t_(A) toT) of the voltage period is zero or at least approaches zero; a currentpath for an excitation current that passes from the voltage source tothe excitation coil and back; an electronic one-way valve with maincurrent electrodes, through which the current path passes so that thecurrent path can be opened and blocked, and with a controlled system toturn the one-way valve on and off; a control circuit that is connectedto the controlled element and so constructed that in order to open thecurrent path the one-way valve is turned on substantially at thebeginning of the first time segment (0 to t_(A)) and to block thecurrent path is turned off at a particular point in time (t₁≦t_(A))within the first time segment (0 to t_(A)), wherein the control circuitis further so constructed that the excitation current does not exceed apredetermined maximal value (I_(max)) and during a time span in whichthe excitation coil is or should be permanently excited, the turning onand off of the one-way valve is repeated in each voltage period (T) toopen and block the current path.
 25. Circuit arrangement according toclaim 24, wherein the voltage source is connected on its input side to asingle-phase mains voltage from the a.c. mains supply and comprises adiode-bridge circuit, so that the first time segment (0 to t_(A))comprises substantially the entire a.c. period (T_(W)) and the secondtime segment (t_(A to t) _(W)) is very short, the period (T)corresponding substantially to half the a.c. period (T_(W)).
 26. Circuitarrangement according to claim 24, wherein the voltage source comprisesa diode-midpoint arrangement, so that the first and the second timesegments (0 to t_(A); t_(A) to T) each correspond to half the voltageperiod (T) and half the a.c. period (T_(W)).
 27. Circuit arrangementaccording to claim 24, wherein a free-run path is disposed parallel tothe terminal connectors of the excitation coil, being so connected thatafter the current path has been blocked, the excitation coil isdischarged with a discharge time constant corresponding to the timeconstant (τ_(B)) of the excitation coil.
 28. Circuit arrangementaccording to claim 24, comprising a voltage meter to monitor theunipolar voltage (U_(G)) and a threshold element to preset a positivethreshold value (U_(G0)), wherein the control device turns on theone-way valve to open the current path when the unipolar voltage(U_(G)), as it rises from zero, first reaches the positive thresholdvalue (U_(G0)), which is considerably lower than the maximal value(U_(Gmax)).
 29. Circuit arrangement according to claim 24, comprising acurrent meter to generate a current signal corresponding to theexcitation current, and a threshold element to preset a first thresholdmaximum (I_(max)) wherein to block the current path the control circuitturns off the one-way valve when the current signal, as it rises fromlower current signals after the current path has been opened, for thefirst time reaches the first threshold maximum (I_(max)).
 30. Circuitarrangement according to claim 29, wherein the control circuit comprisesdevices to alter the threshold maximum (I_(max)) in dependence on anoperating state of the brake while the brake is being raised and whileit is being kept in the raised state.
 31. Circuit arrangement accordingto claim 30, wherein the control circuit comprises a timing element bymeans of which, at the beginning of a first supply of current forraising the brake, the threshold maximum (I_(max), F_(max), U_(max)) isshifted for an elevation interval (T_(E)) to a level higher than thelevel while the brake is being kept in the raised state.
 32. Circuitarrangement according to claim 31, comprising at least one ohmicresistance to supply the timing element with voltage from the unipolarvoltage (U_(G)).
 33. Circuit arrangement according to claim 24,comprising an integrator device to form an integral value, beginning atzero, of the unipolar voltage (U_(G)) over the first time segment (0 tot_(A)) and a threshold element to preset a second threshold maximum(F_(max)) at a level such that the excitation current does not exceed apredetemined maximum (I_(max)), wherein the integral value and thesecond threshold maximum (F_(max)) are sent to the control circuit sothat the one-way valve is turned off and the current path blocked whenthe integral value, rising from lower integral values after the currentpath has been opened, first reaches the second threshold maximum(F_(max)).
 34. Circuit arrangement according to claim 33, comprising atleast one ohmic resistance and a capacitor to form the integratordevice, at least one ohmic resistance to connect the integrator deviceto the unipolar voltage (U_(G)) during the first time segment (0 tot_(A)), and an electronic switch and an ohmic resistance to reset theintegrator device to the value zero in the second time segment (t_(A) toT).
 35. Circuit arrangement according to claim 33, wherein the controlcircuit comprises devices to alter the threshold maximum (F_(max)) independence on an operating state of the brake while the brake is beingraised and while it is being kept in the raised state.
 36. Circuitarrangement according to claim 24, comprising a voltage meter to monitorthe unipolar voltage (U_(G)) and to produce a voltage measurementcorresponding to the unipolar voltage (U_(G)), a first-order delayelement with a time constant that corresponds substantially to theduration of the first time segment (0 to t_(A)) to produce a weightedmeasure from the voltage measurement, and a threshold element to preseta third threshold maximum (U_(max)) at a level such that the excitationcurrent does not exceed a predetemined maximum (I_(max)), wherein toblock the current path the control circuit turns off the one-way valvewhen the weighted measure, rising from a lower weighted measure afterthe current path has been opened, first reaches the third thresholdmaximum (U_(max)).
 37. Circuit arrangement according to claim 36,wherein the first-order delay element comprises at least one ohmicresistance and a capacitor and is connected to the unipolar voltage(U_(G)) by way of at least one ohmic resistance.
 38. Circuit arrangementaccording to claim 36, wherein the control circuit comprises devices toalter the threshold maximum (U_(max)) in dependence on an operatingstate of the brake while the brake is being raised and while it is beingkept in the raised state.
 39. Circuit arrangement according to claim 24,comprising a voltage meter to monitor a voltage at the terminalconnectors of the excitation coil and to produce a correspondingterminal-voltage measurement, a first-order delay element with a timeconstant that corresponds substantially to the time constant (τ_(B)) ofthe excitation coil and preferably is at least twice as long as theperiod (T) of the unipolar voltage (U_(G)), to produce a weighted secondmeasure from the terminal-voltage measurement, and a threshold elementto preset a fourth threshold maximum (U_(max)) at a level such that theexcitation current does not exceed a predetermined maximum (I_(max)) sothat to block the current path the control circuit turns off the one-wayvalve when the weighted second measure, rising from lower values afterthe current path has been opened, first reaches the fourth thresholdmaximum (U_(max)).
 40. Circuit arrangement according to claim 39,comprising at least one ohmic resistance and a capacitor to form thefirst-order delay element, at least one ohmic resistance to connect thefirst-order delay element to the unipolar voltage (U_(G)) when thecurrent path is open, and an electronic switch to short-circuit thefirst-order delay element at its input side when the current path isblocked.
 41. Circuit arrangement according to claim 39, wherein thecontrol circuit comprises devices to alter the threshold maximum(U_(max)) in dependence on an operating state of the brake while thebrake is being raised and while it is being kept in the raised state.42. Circuit arrangement according to claim 24, comprising an overvoltageprotection device that monitors a voltage between the main currentelectrodes and always turns off the one-way valve, to protect it fromtransient voltages, when the voltage temporarily exceeds an overvoltagelimit (U_(Grenz)), which is higher than the maximal value (U_(Gmax)) ofthe unipolar voltage (U_(G)) that is expected in normal operation. 43.Circuit arrangement according to claim 24, comprising a protectivedevice that turns the one-way valve on when a positive voltage isapplied to the controlled element and turns it off when the voltage atthe controlled element is negative or zero.
 44. Circuit arrangementaccording to claim 43, comprising at least one ohmic resistance toconnect the controlled element to the unipolar voltage (U_(G)) in orderto turn on the one-way valve and a limiting bipolar transistor connectedparallel to the controlled element in order to limit a positive controlvoltage.
 45. Circuit arrangement according to claim 43, comprising atleast one ohmic resistance and an electronic switch to short-circuit thecontrolled element when the one-way valve is turned off.